Ignore build artifacts.
Add 'hd' (hexdump) command.

This really needs common/microsd.c's IN_OUT_BUF_SIZE to be a multiple
of 16. Currently it's not, which means the hex dump is badly formatted
at multiples of IN_OUT_BUF_SIZE.
Merge pull request #2 from biot/hd

gitignore and hexdump feature
Document 'hd' option.